Fix issues with opening files from LibreOffice test suite

[Pankraty]

Continuation of #806.

Quote:

Not urgent, but at some stage, we should investigate one by one why these files don't load in ClosedXML. I see we have to do some work on pivot tables. That doesn't surprise me. Maybe you can look at the first file, formats.xlsx, since you've been doing a lot of work on it.

[Pankraty]

From the collection of files that fail to load in ClosedXML

1. 11 files relate to pivot tables and I don't feel brave enough to cope with them
2. 2 files are invalid (formats.xlsx and tdf100709.xlsx). ClosedXML is not designed to manage corrupted files so I suggest removing them.
3. 2 files have comments of non-rectangular shape (universal-content-strict.xlsx and universal-content.xlsx) and wait for XLWorkbook.GetCommentShape returns null and LoadSpreadsheetDocument throws NullReferenceException when comment shape is not rectangle #645.
4. xf_default_values.xlsx has pictures with the duplicate ids. Apparently, duplicate IDs are allowed in XLSX so I removed the validation that threw exceptions on attempts to assign the existing ID to the picture.

[igitur]

I feel that pivot tables should be refactored. The current code is quite messy.